1. Field of the Invention:
The invention resides in the field of polymer fabrication and more particularly relates to methods for manufacturing articles of UHMW polymer.
2. Description of the Prior Art:
Ultra high molecular weight polymers, i.e., those having weights between approximately 1.7 and 6 million are known in the industry for their highly desirable characteristics. These include extremely high resistance to impact, abrasion and chemicals as well as high energy absorption and self-lubrication. They have a high electrical resistance, do not absorb water, and have a useful temperature range from -450.degree. to 280.degree. F. and upward. Exact and detailed specifications of particular polymers are available from suppliers of the material which include "1900", trademarked and produced by Hercules Incorporated, Wilmington, Del.; and "Hostalen GUR", trademarked by and available from American Hoechst Corporation, Somerville, N.J. UHMW polymer is also available from Soltex Polymer Corporation, Houston, Tex.
Three fabrication methods are currently used to form UHMW polymers into semi-finished or stock size configurations. These are compression molding, extrusion, and forging. After initial fabrication, semi-finished and stock shapes may be drilled, die cut, planed, sawed, punched, milled and turned. Basically, the material in solidified form can be worked as though it were a wood or metal. However, because of the material's high molecular weight, it has heretofore been incapable of injection molding. This results from the fact that relative to other thermoplastic material, UHMW polymer has no melt index.
Using a conventional ram type injection heating unit, the material can be injected into a mold, but, since the conventional process requires packing with replacement material as the injected material cools and shrinks, laminated layers result throughout. Further, when using the conventional injection molding method, the material crystallizes with exposure to air, producing an article having a non-uniform, sometimes burned surface having occasional voids and sink marks which render the resulting product useless.
As is well known, injection molding is one of the most efficient and economical methods of fabricating plastics and polymers, in that finished or essentially finished parts of complex shape and design may be produced in great quantity at a rapid rate. The present invention overcomes the past inability to injection mold UHMW polymer through a unique combination of steps which comprise a process not known in the prior art.